Datasheet

Section 23 A/D Converter
Rev. 2.00 Aug. 20, 2008 Page 911 of 1198
REJ09B0403-0200
23.7.6 Notes on Noise Countermeasures
In order to prevent damage due to an abnormal voltage such as an excessive surge at the analog
input pins (AN0 to AN7), a protection circuit should be connected between AVcc and AVss as
shown in figure 23.9. Also, the bypass capacitors connected to AVcc and the filter capacitors
connected to AN0 to AN7 must be connected to AVss.
When a filter capacitor is connected, the input currents at the analog input pins (AN0 to AN7) are
averaged which may cause an error. Also, when A/D conversion is performed frequently, as in
scan mode, if the current charged and discharged by the capacitance of the sample-and-hold circuit
in the A/D converter exceeds the current input via the input impedance (R
in
), an error will arise in
the analog input pin voltage. Therefore, careful consideration is required upon deciding the circuit
constants.
AVcc
*
1
AN0 to AN7
AVss
Notes: Values are reference values.
*
1
*
2 R
in
: Input impedance
*
1
R
in
*
2
100 Ω
0.1 µF
0.01 µF10 µF
AVref
Figure 23.9 Example of Analog Input Protection Circuit
Table 23.6 Standard of Analog Pins
Item Min. Max. Unit
Analog input
capacitance
 20 pF
Acceptable signal
source impedance
 5 kΩ